Clean Water:Harnessing the Power of Plants to Improve Water Quality

Since the Edo period (1604-1868), the outer defensive moat has played a prominent role in the city’s waterfront. However, most sections of the moat have been filled in and today only four retain water: Ushigome Moat, Shinmitsuke Moat, Ichigaya Moat, and Benkei Moat. Unfortunately, the water in these moats has become stagnant due to insufficient water flow, resulting in the decline of quality of the water. Blooms of phytoplankton, specifically blue-green algae, have led to foul odors and unsightly conditions.

To address these issues, the Outer Moat Waterfront Regeneration Council has spearheaded water quality improvement initiatives. The council not only conducts its own research and experiments, it also supports outside academic studies and organizes public events. Our findings are shared with the Tokyo Metropolitan Government, Chiyoda-ward, and various other organizations. Our results are also presented at academic conferences.

One key initiative to restore the water quality in the moats is via aquatic plants. In this article, we explore the natural purifying power of underwater plants.

The Purifying Power of Aquatic Plants

Aquatic plants grow entirely underwater and are rooted in the sediment. They are known to purify water as well as provide habitats for aquatic life. Similar initiatives using underwater plants to improve water quality have been implemented in lakes and ponds across Japan.

Of the four moats that still retain water—Ushigome Moat, Shinmitsuke Moat, Ichigaya Moat, and Benkei Moat—the latter has the best water quality and supports the healthy growth of submerged plants. One such plant, Myriophyllum spicatum, has been proven in other studies to help suppress the growth of blue-green algae. Based on these results, the Council believes this species may be the key to improving water quality throughout the peripheral moats.

Can Myriophyllum spicatum grow in the Ushigome, Shinmitsuke, and Ichigaya Moats?

Myriophyllum spicatum, also known in Japanese as kingyomo, is a native, perennial aquatic plant that grows throughout Japan, from Hokkaido to Kyushu. While it grows abundantly in Benkei Moat, it has not been observed in recent years in the other, more northern moats of Ushigome, Shinmitsuke, and Ichigaya.

However, soil surveys conducted in the Sotobori Park along the banks of the moats have uncovered evidence that members of the Myriophyllum genus once grew there. This suggests that Myriophyllum spicatum likely thrived in the northern moats during the Edo period.

Based on this information, it seems plausible that the plant could be reintroduced into those moats. If successfully reestablished, Myriophyllum spicatum could help improve water quality of all the moats. Since no case studies have yet reported on using the plant for water purification in the outer moat, the Council decided to launch its own experiment to determine how effectively Myriophyllum spicatum can suppress blue-green algae.

Water Purification Experiment Using Myriophyllum spicatum

The Council conducted the following experiment in which four types of aquarium setups were prepared:

One aquarium containing both blue-green algae collected from Ichigaya Moat and Myriophyllum spicatum collected from Benkei Moat.
One acquaium planted with blue-green algae only (no Myriophyllum spicatum)
Two aquariums planted with with varying amounts of the Myriophyllum spicatum plant.

Each condition was replicated three times to account for variability, with a total of 12 tanks. The experiment lasted 60 days.

In tank without any Myriophyllum spicatum, blue-green algae bloomed. Furthermore, this tanks showed increased concentrations of phosphorus and other substances known to degrade water quality. However, in those tanks with aquatic plants, no algea blooms were observed and concentrations of phosphorus and other harmful substances remained lower. Since phosphorus serves as a nutrient source for blue-green algae, its lower concentration helps suppress algae growth.

This experiment confirmed that Myriophyllum spicatum growing in Benkei Moat, can effectively inhibit the growth of algae in the Ichigaya Moat.

Toward Cleaner Water Through Aquatic Plants

The discovery that Myriophyllum spicatum growing in Benkei Moat helps suppress the growth of blue-green algae offers new hope for improving water quality throughout the entire moat system. By transplanting the thriving plants from Benkei Moat into other moats—such as Ichigaya—with poorer water conditions, further water quality improvement is anticipated.

The blue-green algae currently found in some of the moats not only produces unpleasant odors; it also disrupts the habitats of other organisms, which raises concerns about the overall ecological balance. Water purification efforts using aquatic plants is therefore expected to help restore this balance.

All algae and aquatic plant samples used in the experiment were collected with prior approval from Chiyoda ward and the Tokyo Metropolitan Government, which manage the outer moat. The Council has shared our results with them.

The Outer Moat Waterfront Regeneration Council will continue its research into water quality and the effects of aquatic plants, consistently providing the latest findings to relevant agencies and contibruting to the ongoing restoration of these waterways.